The rapid development of computers has established the information industry of today and has been accompanied by very active research into methods for outputting massive amounts of records. Silver halide photographic materials that are suitable for reversal processing are used as recording materials in this field. Processing in this reversal development consists of forming a negative image by a first development process, and not subjecting it to fixing but to bleaching to desilver the reduced silver in the negative image. The remaining undeveloped image silver halide is exposed and a second development is performed to produce a positive image. Because of the complexity of the processing stages, the film finishing rate is slow and there are fluctuations in the maximum density (Dmax) and minimum density (Dmin). In addition, there are problems of environmental pollution since powerful oxidizing agents such as potassium dichromate have to be used in the bleaching solution.
Photographic methods for producing direct positive images without a reversal processing stage or a negative film are well-known methods for resolving these problems.
From the point of view of practical use, with the exception of special methods, these conventional methods for producing positive images using direct positive silver halide photographic materials can be classified into two types.
One type uses a prefogged silver halide emulsion and a direct positive image is obtained after development by using solarization or the Herschel effect, to break down the fogging nuclei (the latent image) of exposed portions.
The other type uses a non-fogged internal latent image type silver halide emulsion and direct positive images are obtained by performing surface development while effecting fogging treatment or after effecting a fogging treatment, following image exposure.
This "internal latent image type silver halide emulsion" is a silver halide emulsion in which the silver halide grains have photosensitive nuclei mainly in their interiors and exposure results in formation of a latent image mainly inside the grains.
The second procedure gives greater speeds than the first type and is more suited to applications where high speeds are demanded, and the present invention relates to the internal latent image type silver halide emulsion.
A variety of techniques are known in this technical field, principal examples being disclosed in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 2,497,875, 3,761,266, 3,761,276 and 3,796,577 and British Patents 1,151,363, 1,150,553 and 1,011,062.
These known methods make it possible to produce a photosensitive material with comparatively high speed as a direct positive type material.
Details of the mechanism of direct positive image formation are given in, e.g., The Theory of the Photographic Process by T. H. James, (4th edition), Chapter 7, pages 182-193 and U.S. Pat. No. 3,761,276.
It is believed that as the result of a surface desensitization action originating in an internal latent image produced inside the silver halide in response to initial image exposure, there occurs selective formation of fogging nuclei only on the surfaces of the silver halide grains in unexposed portions, and that following this by ordinary surface development processing results in formation of a photographic image (direct positive image) in the unexposed portions.
Methods which are generally called "light fogging methods" in which the entirety of photosensitive surfaces are subjected to a second exposure (e.g., British Patent 1,151,363) and methods called "chemical fogging methods" using nucleating agents are known as means for selectively producing fogging nuclei as noted above. A description of the chemical fogging method is given, e.g., at pages 72-87 of Research Disclosure Volume 151, No. 15162 (published November 1976).
Materials with which nucleating agent effects are achieved only at a high pH of 12 or more are used in conventional chemical fogging methods, but deterioration of the developing agent due to air oxidation is liable to occur in these high pH conditions and consequently there is the drawback that the development activity is extremely reduced. There is also the drawback that processing takes a long time because the development rate is slow, and in particular the processing takes even longer if a low pH developing solution is used. There is also the drawback that processing takes a long time even if the pH is increased to 12 or more.
In contrast, light fogging methods are comparatively advantageous for practical uses since they do not need high pH conditions. On the other hand, there are various technical problems if a variety of technical purposes are to be served over a broad photographic field. That is, since a light fogging method is based on formation of fogging nuclei through photolysis of silver halide, the appropriate illuminance and quantity of exposure vary depending on the type and characteristics of the silver halide used. This means that there are the drawbacks that it is difficult to achieve constant performance and that the development apparatus is complex and costly. There is the further drawback that development takes a long time.
It has been found difficult to produce good, stable direct positive images in both these conventional types of fogging methods. Compounds which display a nucleating action even at a pH of 12 or less have been proposed in JP-A-52-69613 (the term "JP-A" used herein means an "unexamined published Japanese patent application") and U.S. Pat. Nos. 3,615,615 and 3,850,638 as means for resolving this problem but these nucleating agents have the drawbacks that they act on silver halide during storage of sensitive material prior to processing and that they themselves are decomposed, leading to a fall in the post-processing maximum image density.
It has been disclosed in U.S. Pat. No. 3,227,552 that intermediate density development rates are increased by use of hydroquinone derivatives. However, even when these derivatives are used the development rate is still insufficient and with a developing solution with a pH of 12 or less only an unsatisfactory development rate is obtained.
JP-A-60-170843 discloses addition of mercapto compounds containing carboxylic acid groups or sulfonic acid groups to increase the maximum image density, but the effects of adding these compounds are slight.
JP-A-55-134848 discloses that processing in a processing solution (pH 12.0) containing tetraazaindene compounds in the presence of a nucleating agent reduces the minimum image density and prevents formation of a re-reversal negative image, but it is not possible to achieve a high maximum image density or quick development rates with this method.
JP-B-45-12709 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses addition of triazolinethione compounds and tetrazolinethione compounds as antifoggants to sensitive materials for forming direct positive images by a light fogging process, but it is not possible to achieve high maximum image density or a rapid development rate with this method.
Thus, conventional techniques do not permit direct positive images with high maximum image density and low minimum image density to be produced in a short time.
There is also the problem that, in general, the higher the speed of a direct positive emulsion, the greater is the occurrence of re-reversal negative images in high illuminance exposure. For COM film in particular, high speed with short CRT exposure is demanded, and so it is important to prevent re-reversal negative images on high illuminance exposure.
Japanese Patent Applications 61-136949 and 61-153481 disclose techniques for resolving the above noted problems but these procedures are unsatisfactory in respect of prevention of re-reversal negative images.